ATRAC

ATRAC (Adaptive Transform Acoustic Coding) is a proprietary audio compression technology developed by Sony for usage in their MiniDisc line of digital audio players.

It actually relates to three different audio compression technologies related among themselves, but with incompatibilities. It uses quadrature mirror filters and modified discrete cosine transform to represent encoded audio.

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ATRAC (also called ATRAC1) was developed in 1991, and used in the first MiniDisc players in the early nineties. It used bitrates of 292kbps for stereo audio and 146kbps for mono. Several subversions were released, from 1 to 4.5, and all of them are backwards- and forwards-compatible. It's reported that it only started sounding good after version 3.